Optical imaging systems, such as digital imaging telescopes, can provide different optical paths to direct light for different applications. FIG. 1A schematically illustrates a typical prior art digital imaging telescope 10. Digital imaging telescope 10 has a telescope body 12 which defines a primary optical cavity 13. Primary optical cavity 13 houses an objective lens system 14, an erecting prism system 16 and an eyepiece 18. Eyepiece 18 houses an eyepiece lens system 20 which provides a magnified image to a user's eye. Telescope body 12 also defines an optical cavity 24 which houses a secondary objective lens 22 and a digital imaging system 26. Digital imaging system 26 is typically implemented using charge-coupled-device (CCD) technology or complementary metal-oxide semiconductor (CMOS) technology.
Digital imaging telescope 10 provides two light paths 30, 32. Light on path 30 travels through objective lens 14 into primary optical cavity 13 and is focused at point F1 in front of eyepiece lens system 20. Light on path 30 allows users to view objects through eyepiece 18. Light on path 32 travels through objective lens 22 into secondary optical cavity 24 and is focused at point F2 to provide light to digital imaging system 26. Digital imaging system 26 digitizes images to generate digital image data.
Digital imaging telescope 10 suffers from the drawback that it requires two objective lenses 14, 22, each having relatively low f/#s which tends to be expensive. Also, the characteristics of objective lens 22 (e.g. aperture, focal length and f/#) and the optical path length of light path 32 are different from the characteristics of objective lens 14 and the optical path length of light path 30. Consequently, the images digitized by digital imaging system 26 differ from the images observed by a user through eyepiece 18. Furthermore, the digital magnification range provided by the digital zoom of digital imaging system 26 is limited by the focal length of secondary objective lens 22. For example, if the focal length of secondary lens 22 is 100 mm and the digital zoom capability of digital imaging system 26 is 5×, then the total digital magnification range of digital imaging system 26 is equivalent to that of an objective lens having a focal length in a range of 100 mm-500 mm.
FIG. 1B schematically illustrates a different prior art digital imaging telescope 110. Digital imaging telescope 110 is similar to digital imaging telescope 10, except that digital imaging telescope 110 uses a single objective lens 114 to provide light to eyepiece 118 and to digital imaging system 126. This light sharing is accomplished in digital imaging telescope 110 by a beam splitter 134 and a flat mirror 128. Beam splitters are well known optical devices and can be fabricated by providing a partially reflective coating between a pair of triangular prisms. Light from optical path 130 which impinges on beam splitter 134 is split into a first component 130A, which continues onward through erecting prism 116 to eyepiece 118 (point F1), and a second component 130B, which emerges from beam splitter 134 at a 90° angle, reflects from flat mirror 128 and then travels to digital imaging system 126 (point F2).
While beam splitter 134 eliminates the need for a second objective lens, the introduction of beam splitter 134 splits the light that is collected by objective lens 114 (i.e. into components 130A, 130B). Consequently, components 130A and 130B each contain only a fraction of the light that is collected by objective lens 114. For a given objective lens 114, the introduction of beam splitter 134 reduces the amount of light available at eyepiece 118, which in turn reduces brightness, contrast and resolution.
Because the index of refraction of the glass used in beam splitter 134 (nglass) is greater than that of air (nair≈1), the introduction and/or withdrawal of beam splitter 134 into light path 130 also changes the optical path length of light traveling between objective lens 114 and eyepiece 118 and between objective lens 114 and digital imaging system 126. Assuming that the thickness of an optical path through beam splitter 134 is L, then the optical path length through beam splitter 134 will be OPLsplitter=L/nglass and the reduction in optical path length caused by the introduction of beam splitter 134 will be ΔOPL=L−L/nglass. Assuming nglass=1.5, this results in OPLsplitter=0.67L or a reduction in optical path length of ΔOPL=L−0.67L=0.33L.
Furthermore, telescope 110 suffers from the same limitations as telescope 10 with respect to its digital magnification range. The digital magnification range provided by the digital zoom of digital imaging system 126 is limited by the focal length of objective lens 114. Another drawback with telescope 110 is that the partially reflective surface of beam splitter 134 can cause a loss of true color and scattering which can induce light decay.
There is a general desire to provide optical imaging systems which address or ameliorate some of the disadvantages discussed above.